Dielectric gelling composition, the use of such dielectric gelling composition, an insulated electric dc-cable comprising such gelling composition, and a method for manufacturing an insulated electric dc-cable comprising such gelling composition

ABSTRACT

Disclosed is a dielectric gelling composition, exhibiting a thermo-reversible liquid-gel transition at a transition temperature, T t , wherein the gel comprises an oil and a combined gelator system having molecules of a polymer compound together with fine dielectric particles with a particle size in the nanomneter, 6 nm, range, preferably a particle size within the range from 0.001 to 1000 nm, the use of this dielectric gelling composition in an electric device comprising one or more conductors, a casing or enclosure and an insulation system comprising the dielectric gelling composition. An electric DC-cable having a conductor and an electrical insulation comprising a solid part with a porous, fibrous and/or laminated structure impregnated with the dielectric gelling composition and a method for production of such DC-cable wherein the combined gelator is prepared prior to impregnation are also disclosed.

TECHNICAL FIELD

The present invention relates to a dielectric gelling compositioncomprising a dielectric fluid and a gelling additive, in particular anelectrical insulation oil to which one or more gelling additives,gelators, i.e. compounds that impart a gelling behaviour in thedielectric fluid, have been added. In particular the invention relatesto such a gelling composition exhibiting a thermo-reversible transitionbetween the easy flowing fluid state at high temperatures and a highlyviscous and elastic gelled state at low temperatures, athermo-reversible liquid-gel transition.

The present invention relates in another aspect to the use of such agelling composition as part of an electrical insulation system for anelectric device.

In a particular aspect the present invention relates to an insulatedelectric direct current cable, an insulated DC-cable, with an insulationsystem comprising such a dielectric gel with a thermo-reversibleliquid-gel transition. The present invention also relates to a methodfor manufacturing such DC-cable. The insulated DC cable is suited fortransmission and distribution of electric power. The insulation systemcomprises a plurality of functional layers, such as an innersemi-conductive shield, an insulation and an outer semi-conductiveshield, wherein at least the insulation comprises a porous, fibrousand/or laminated body impregnated with a dielectric fluid.

BACKGROUND ART

Electrical insulation oils and other dielectric fluids are used inelectric insulation systems for devices such as transformers,capacitors, reactors, cables and the like. The dielectric fluids aretypically used in combination with a porous, fibrous and or laminatedsolid part, which is impregnated with the dielectric fluid, the electricinsulating oil, but also as encapsulants to prevent water penetration.The active part of an impregnated insulation is the solid part. The oilprotects the insulation against moisture pick-up and fills all pores,voids or other interstices, whereby any dielectrically weak air in theinsulation is replaced by the oil. Impregnation is typically a timeconsuming and delicate process carried out after the solid part of theinsulation has been applied and needs to be carefully monitored andcontrolled. For example, the impregnation of a DC-cable intended for along distance transmission of electric power, where several kilometresof a cable are treated, typically exhibits a process cycle timeextending over days or weeks or even months. In addition, this timeconsuming impregnation process is made according to a carefullydeveloped and strictly controlled process cycle with specified rampingof both temperature and pressure conditions in the impregnation vesselused during heating, holding and cooling to ensure a complete and evenimpregnation of the fiber-based insulation. The impregnation of otherinsulation systems comprising dielectric fluids such as transformers,capacitors and the like is, although not as time consuming as theimpregnation of a DC-cable, a sensitive process and specific demands areput on the impregnant, the medium to be impregnated and the processvariables used for impregnation.

To ensure a good impregnation result, a fluid exhibiting a low-viscosityis desired. The fluid shall also preferably be viscous at operationconditions for the electrical device to avoid migration of the fluid inthe porous insulation. Darcy's law (1) is often used to describe theflow of a fluid through a porous or capillary medium. (1):  $v = \frac{k\quad \Delta \quad P}{\mu \quad L}$

In this law v is the so called Darcy velocity of the fluid, defined asthe volume flow divided by the sample area, k is the permeability of theporous medium, ΔP is the pressure difference across the sample, μ is thedynamical viscosity of the fluid and L is the thickness of the sample.The flow velocity of a fluid within a porous medium is essentiallyreciprocally proportional to the viscosity. A fluid exhibiting alow-viscosity or a highly temperature dependent viscosity at operatingtemperature will have a tendency to migrate under the influence oftemperature fluctuations naturally occurring in an electric deviceduring operation and also due to any temperature gradient building upacross a conductor insulation in operation and might result in unfilledvoids being formed in the insulation. Temperature fluctuations andtemperature gradients are present in a high-voltage DC cable, and thusany problem associated with migration of the dielectric fluid must becarefully considered. Unfilled voids or other unfilled interstices orpores in an insulation operating under an electrical high-voltage directcurrent field constitute deficiencies where space charges tend toaccumulate. Accumulated space charges might under unfavorable conditionsinitiate dielectric breakdown through discharges which will degrade theinsulation and ultimately might lead to its breakdown. The idealdielectric fluid should exhibit a low-viscosity under impregnation andbe highly viscous under operation conditions.

Conventional dielectric oils used for impregnating a porous, fibrous orlaminated conductor insulation in an electric device such as a DC cableexhibit a viscosity that decreases essentially exponential as thetemperature increases. The impregnation temperature must therefore besubstantially higher than the operation temperature to gain the requireddecrease in viscosity due to the low temperature dependence of theviscosity at high temperatures. In comparison, the temperaturedependence of the viscosity at temperatures prevailing during operationconditions is high. Small variations in impregnation or operationconditions affect the performance of the dielectric fluid and theconductor insulation. Oils are therefore selected such that they aresufficiently viscous at expected operation temperatures to beessentially fully retained in the insulation also under the temperaturefluctuations that occur in the electric device during operation. Theretention shall also be essentially unaffected of any temperaturegradient building up over an insulation. This typically leads to a highimpregnation temperature being used to ensure that the insulation willbe essentially fully impregnated. However, a high impregnationtemperature is disadvantageous as it risks effecting the insulationmaterial, the surface properties of the conductor, and promotingchemical reactions within and between any material present in the devicebeing impregnated. Also energy consumption during production and overallproduction costs are negatively affected by a high impregnationtemperature. Another aspect to consider is the thermal expansion andshrinkage of the insulation which implies that the cooling must becontrolled and slow, adding further time and complexity to an alreadytime consuming and complex process. Other types of oil impregnatedcables employ a low viscosity oil. However, these cables then comprisetanks or reservoirs along-the cable or associated with the cable toensure that the cable insulation remains fully impregnated upon thermalcycling experienced during operation. With these cables, filled with alow viscosity oil, there is a risk for oil spillage from a damagedcable. Therefore an oil exhibiting a highly temperature dependentviscosity and with a high viscosity at operating temperature ispreferred.

To impart a suitable increased temperature dependency in the viscosityfor a conventional mineral oil, it is known to add and dissolve apolymer, e.g. polyisobuthene, in the oil. This can only be achieved forhighly aromatic oils, but oils of this kind typically exhibit, poorerelectric properties in comparison with more naphtenic oils. These latterare oil types suitable for use in electric insulations. A more aromaticoil must typically be treated with bleaching earth to exhibit acceptableelectric properties. Such processing is costly and there is a risk thatsmall sized clay-particles remain in the oil if not a careful filter- orseparation-processing is carried out after this treatment.Alternatively, an oil as disclosed in U.S. Pat. No. 3,668,128 comprisingadditions of from 1 up to 50 percent by weight of an alkene polymer witha molecular weight in the range 100-900 derived from an alkene with 3, 4or 5 carbon atoms, e.g. polybutene, can be chosen for its low viscosityat low temperatures. This oil exhibits a low viscosity at lowtemperatures, good oxidation resistance and also good resistance togassing, i.e. the evolution of hydrogen gas which might occur,especially when an oil of low aromatic content, as the oil suggested inU.S. Pat. No. 3,668,128, is exposed to electrical fields. However, theoil according to the disclosure in U.S. Pat. No. 1,668,128, althoughoffering a major advance on the traditional electrical insulating oilfor impregnation of fibrous or laminated insulations, still suffers therisk of oil migration caused by temperature fluctuations and/ortemperature gradients building up under operation as the low viscosityoil is typically not retained during operation at elevated temperatures.

The earlier not yet published International Patent ApplicationPCT/SE97/01095 discloses a DC-cable impregnated with a gellingdielectric fluid, such as an oil. The dielectric fluid comprises agelling polymer additive that imparts to the fluid a thermo-reversibletransition between a gelled state at low temperatures and an essentiallyNewtonian easy flowing state at high temperatures. This substantialtransition in viscosity occurs over a limited temperature range. Thefluid and the gelling polymer additive are matched to impart athermo-reversible gelling behavior with a liquid-gel transition range tothe fluid to suit the desired properties both during impregnation andoperation. The fluid is, at high temperatures, in a liquid state andexhibits the viscosity of an easy flowing Newtonian fluid. At lowtemperatures the fluid is in a gelled state, with a viscosity of ahighly viscous, elastic gel. The transition temperature is determined bythe selection of fluid and additive and the content of additive. Such acable exhibits a substantial potential for reduction of the time periodneeded for impregnation but it still requires a strictly controlledtemperature cycle during impregnation. The gelling polymer additive andthe dielectric fluid are matched or optimized to, in the best way, meetthe typically conflicting demands during impregnation and use of thecable. There is in the art a strong desire to reduce impregnationtemperatures and at the same to increase the current densities in theDC-cables. Increased current densities will while using same conductorsand same conductor dimensions lead to increased operation temperaturesin the DC-cable. Meeting both these conflicting demands will furtherreduce the gap between the impregnation temperature and operationtemperature. Consequently, it will be harder to match the specificdemands even with sophisticated gelling systems. It must be rememberedthat not only shall essentially all voids and interstices of the cableinsulation be filled by the fluid but the fluid shall also be retainedin this insulation as the temperature fluctuates and temperaturegradients build up during operation. Suitable gelling systems,comprising oils and polymers, for other purposes are discussed in theEuropean Patent Publication EP-A1-0 231 402. This publication disclosesa gel-forming compound with slow forming and thermally reversiblegelling properties intended to be used as an encapsulant to ensure agood sealing and blocking of any interstices in a cable comprising anall solid insulation, such as an extruded polymer based insulation. Theslow-forming thermally reversible gelling compound comprises anadmixture of a polymer to a naphtenic or paraffinic oil, and alsoembodiments using further admixtures of a co-monomer and/or a blockcopolymer to an oil are considered suitable as encapsulant due to theirhydrofobic nature and the fact that they can be pumped into theinterstices at a temperature below the maximum service temperature ofthe encapsulant itself. Similar gel-forming compounds for the samepurpose, i.e. the use as encapsulant to block water from entering andspreading longitudinal in a cable are also known from the EuropeanPatent Publications, EP-A1-0 058 022 and EP-A1-0 586 158.

Thus, it is desirous to provide a dielectric gelling composition with athermo-reversible liquid-gel transition at a high temperature, andwithin a narrow temperature range. The gelling composition shall exhibitproperties whereby the impregnation can be enhanced and the impregnationtime shortened. It shall exhibit a high viscosity at the temperaturerange within which the device is designed to operate, thereby reducingthe risks for migration and formation of voids upon thermal cyclingand/or under thermal gradients. The volume changes upon thermal cyclingshall be reduced. In particular importantly, the shrinkage upon coolingafter impregnation and any problems associated with such shrinkage shallbe reduced. Further, the gelling composition shall exhibit such thermal,mechanical and electric properties and stability in these propertiessuch that it opens for an increase in load, i.e. an increase in bothoperation voltages and current densities used in the device.

Many of the first electrical supply systems for transmission anddistribution of electrical power were based on DC technology. However,these DC systems were rapidly superseded by systems using alternatingcurrent, AC. The AC systems had the desirable feature of easytransformation between generation, transmission and distributionvoltages. The development of modern electrical supply systems in thefirst half of this century was exclusively based on AC transmissionsystems. By the 1950s there was a growing demand for long transmissionschemes and it became clear that in certain circumstances there could bebenefits by adopting a DC based system. The foreseen advantages includea reduction of problems encountered in association with the stability ofthe AC-systems, a more effective use of equipment as the power factor ofthe system is always unity and an ability to use a given insulationthickness or clearance at a higher operating voltage. Against these verysignificant advantages has to be weighed the cost of the terminalequipment for conversion of the AC to DC and for inversion of the DCback again to AC. However, for a given transmission power, the terminalcosts are constant and therefore, DC transmission systems are economicalfor schemes involving long distances, such as for systems intended fortransmission from distant power plants to consumers but also fortransmission to islands and other schemes with transmission distanceswhere the savings in the transmission equipment exceed the cost of theterminal plant. An important benefit of DC operation is the virtualelimination of dielectric losses, thereby offering a considerable gainin efficiency and savings in equipment. The DC leakage current is ofsuch small magnitude that it can be ignored in current ratingcalculations, whereas in AC cables dielectric losses cause a significantreduction in current rating. This is of considerable importance forhigher system voltages. Similarly, high capacitance is not a penalty inDC cables. A typical DC-transmission cable includes a conductor and aninsulation system comprising a plurality of layers, such as an innersemi-conductive shield, an insulation body and an outer semi-conductiveshield. The cable is typically complemented with casing, reinforcement,etc., to withstand water penetration and any mechanical wear or forcesduring production, installation and use: Almost all the DC cable systemssupplied so far have been for submarine crossings or the land cableassociated with them. For long crossings the mass-impregnated solidpaper insulated type of cable is chosen because there are norestrictions on length due to pressurizing requirements. It has to datebeen supplied for operating voltages of 450 kV. These voltages arelikely to be increased in the near future. To date an essentially allpaper insulation body impregnated with an electric insulation oil hasbeen used, but application of laminated material such as a polypropylenepaper laminate is being persued. As in the case of AC transmissioncables, transient voltages is a factor that has to be taken into accountwhen determining the insulation thickness of DC cables. It has beenfound that the most onerous condition occurs when a transient voltage ofopposite polarity to the operating voltage is imposed on the system whenthe cable is carrying full load. If the cable is connected to anoverhead line system, such a condition usually occurs as a result oflightning transients. A commercially available insulated electricDC-cable such as a transmission or distribution cable designed foroperation at a high voltage, i.e. a voltage above 100 kV, is typicallymanufactured by a process comprising the winding or spinning of aporous, fibrous and/or laminated solid insulation based on cellulose orpaper fiber, and the impregnation of this cable. The impregnationprocess, the times and controlled processing involved have already beendescribed in the foregoing.

Thus it is desirous to provide an insulated DC-cable with an electricalinsulation system that ensures stable dielectric properties also whenoperating at high operation temperatures close to the impregnationtemperature and/or under conditions where the insulation duringoperation is subjected to a high voltage direct current field incombination with thermal fluctuations and/or a build up of a substantialthermal gradient within the insulation. The dielectric fluid employedshall exhibit a high viscosity index such that it during impregnationhas a sufficiently low viscosity, i.e. a viscosity deemed suitable andtechnically and economically favorable for impregnation, and that itafter impregnation has a high viscosity and elasticity, i.e. a viscositythat ensures that it during operation, will be essentially retained inthe porous, fibrous and/or laminated insulation body at all temperatureswithin the range of temperatures for which the DC-cable is designed tooperate. The DC-cable shall thus comprise a dielectric fluid with asufficiently low viscosity prior to and during impregnation to ensurestable flow properties and flow behavior within these ranges, and whichexhibits a substantial change in viscosity upon impregnation, i.e. achange in the order of hundreds of Pas or more. A DC-cable impregnatedwith a fluid exhibiting such high viscosity index will provide anopportunity for a substantial reduction in the lengthy time consumingbatch-treatment for impregnation of the insulation system, therebyproviding a potential for a substantial reduction in the production timeand thus the production costs. The reliability, low maintenancerequirements and long working life of conventional DC-cables, comprisingan impregnated paper-based insulation shall be maintained or improved.That is, the DC-cable shall have stable and consistent dielectricproperties and a high and consistent electric strength and, as an extraadvantage, open for an increase in the electrical strength and thusallow an increase in operation voltages, improved handleability androbustness of the cable.

SUMMARY OF THE INVENTION

According to the present invention it is an object to provide adielectric gel, which exhibits a thermo-reversible liquid-gel transitionat a high temperature with the desirous features discussed in theforegoing. This is for a dielectric gel according to the preamble ofclaim 1 accomplished by the features of the characterizing part of claim1. Further developments of the dielectric gel according to the presentinvention are characterized by the features of the additional claims 2to 25. It is also an object to provide the use of such a gel in electricdevices. This is accomplished according to claim 26 to 28. In particularits an object of the present invention to provide an insulated electricdevice comprising such a dielectric gel as impregnant in its impregnatedinsulation system. This is for a device according to the preamble ofclaim 29 accomplished by the features of claim 29. Further developmentsof the DC-cable according to the present invention are characterized bythe features of the additional claims 30-38. Further claims 39 to 49define a method for manufacturing an electric device according to thepresent invention.

DESCRIPTION OF THE INVENTION

The primary object is accomplished with a dielectric gellingcomposition, exhibiting a thermo-reversible liquid-gel transition at atransition temperature, T_(t), wherein the gel comprises an oil and agelator, which according to the present invention comprises a combinedgelator system having molecules of a polymer compound together with finedielectric particles with a particle size in the nanometer, nm, range,preferably a particle size of 1000 nm or less. Suitably, a particle sizeof from 1 to 1000 nm, and preferably within the range of from 10 to 100nm. The dielectric gelling composition which comprises an oil and agelator exhibits a thermo-reversible liquid-gel transition at atransition temperature, T_(t), wherein the gelling composition attemperatures below T_(t) is in a highly viscous elastic gelled statedand, at temperatures above T_(t), is in a liquid easy flowingessentially Newtonian state. The polymer and the oil interact to developa three dimensional, physically cross-linked gelled network attemperatures below the transition temperature T_(t). Typically, thetransition temperature T_(t) is a narrow range of temperatures above 50°C., preferably of from 70° C. to 150° C. Thus, the gelled network oflonger and/or more branched polymer molecules or cross-linking bridgesin the oil formed through the gelling interaction between the combinedgelator and the oil is characterized by the physical bonds developed.The network will increase the viscosity index of the oil such that thegelled network in the oil according to the present invention attemperatures below the transition temperature T_(t) exhibits theproperties of an elastic gel.

According to one embodiment the fine particles are trapped within thegelled network of polymer. The particles can either be mechanicallylocked in the network or physically bonded to the gelled network ofpolymer. Alternatively, the polymer molecules are grafted onto the fineparticles, but also blends with other types of physical and chemicalbonds can be adequate depending on the nature of the particle, thepolymer molecule and the oil. The fine particles are preferably evenlydistributed within the gelled network and provide a reinforcement of thegelled network and the insulation system. The reinforcement is bothelectrical and mechanical. Another advantage of the combined gelatorsystems used according to the present invention is that their gellingkinetics can be modified which opens for a delayed significantly slowergelling if so desired, this delay can in some cases exceed 24 h.

According to one embodiment the dielectric gelling composition comprisessilica The gelling composition can also comprise other dielectricinorganic particles with suitable electric and thermal properties suchas alumina, zirconia, calcia and other oxides, silicon nitride,electrically insulating forms of carbon, zeolites, unexpanded andexpanded mica, clays, talcs and the like. The particles can also becoated with any of the materials mentioned in the foregoing, wherein thecoating can be applied also on metallic materials, e.g. fine particlesof titanium coated with silica. The fine dielectric particles can alsocomprise organic materials, such as cellulose based materials, e.g.cellulose powder or micro-crystalline cellulose. Typically, thedielectric fluid is an electrical insulation oil to which variousgelling additives have been added. Generally, suitable gelling additivesfor most types of oils are compounds such as;

a compound compromising a polar segment that has a tendency to develophydrogen bonds, preferably compounds comprising polar segments and longnon-polar hydrocarbon chains,

sugar based compounds,

compounds comprising urea or di-urea,

a compound comprising a block copolymer.

Polymeric compounds as described in the earlier not yet publishedInternational Patent Application PCT/SE97/01095 can advantageously beused for at least any dielectric fluid based on a mineral oil. Gellingadditives comprising a polyalkylsiloxane are well suited at least for adielectric fluid based on a silicone oil, while gelling additivescomprising a cellulose based compound are suitable for at least anydielectric fluid based on a vegetabilic oil. According to one embodimentthe gelling composition also comprises an addition of a surfactant tofurther enhance impregnation.

A gelling dielectric composition as described in the foregoingcomprising oil and a combined gelator system having molecules of apolymer compound together with fine dielectric particles is suitable foruse as part of an insulation system in an electric device comprising oneor more conductors. Due to the dielectric particles dispersed in theelastic gel of the composition after gelling, an insulation systemconsisting of a gelled body only comprising dielectric gellingcomposition can be contemplated, provided that the amount and volume ofthe dielectric particles are sufficient. According to a preferredembodiment the dielectric gelling composition is included as impregnantin an insulation system comprising a porous, fibrous and/or laminateddielectric body impregnated with the dielectric gelling composition,such as the insulation system in a cable, a transformer or thedielectric between the electrodes in a capacitor. Here it is anadvantage that the gelling kinetics of the combined gelator systems usedaccording to the present invention can be modified, which opens for adelayed significantly slower gelling if so desired, this delay can insome cases exceed 24 h. This results in a decreased shrinkage when aninsulation comprising a gelling impregnant in the form of the gellingcomposition according to the invention is used. As a consequence, the“post-filling” step is less critical.

A DC-cable having at least one conductor and an impregnated insulationsystem, wherein the insulation system comprises a solid electricallyinsulating dielectric part with a porous, fibrous and/or laminatedstructure impregnated with a dielectric gelling composition, whichaccording to the present invention comprises oil and the combinedgelator system having molecules of a polymer compound together with finedielectric particles, meets the object set out according to the aspectof the present invention relating to an insulated DC-cable. Preferablythe dielectric gelling composition comprises a mineral oil and acombined gelator system comprising dielectric particles with a particlesize in the nanometer range and molecules of a polymer compound. Thepolymer molecules can be grafted onto the fine particles, but alsoblends with other types of physical and chemicals bonds can be adequatedepending on the nature of the particle, the polymer molecule and theoil. Also systems were the particles are trapped in the gelled networkupon formation of the gelled network following cooling to a temperaturebelow T_(t), are advantageous and provide a reinforcement andstabilization of the gelled network and the total insulation system. Thecomponents within the dielectric gelling composition and the oilinteract to develop a three dimensional, physically cross-linked networkat temperatures below the transition temperature T_(t). Typically, thetransition temperature T_(t) is a narrow range of temperatures above 30° C., preferably within the range of from 50 ° C. to 120° C. Accordingto one embodiment the dielectric gelling composition is selected suchthat it interacts with the surface of the porous, fibrous and/orlaminated structure, wherein the interaction between the dielectricgelling composition and the surface of the porous, fibrous and/orlaminated structure either can provide conditions that increase the oilpenetration into voids and capillary interstices within the porous,fibrous and/or laminated structure upon filling, or that increase theoil retention within the porous, fibrous and/or laminated structure uponoperation at a high temperature, fluctuating temperatures and/or under asubstantial temperature gradient. Thus, depending on its nature theinteraction with the solid parts of the insulation can result in animproved wetting which shortens the impregnation time period due to anincrease in the oil penetration into voids and capillary intersticeswithin the porous, fibrous and/or laminated structure upon filling. Theinteraction can also under other circumstances increase the oilretention within the porous, fibrous and/or laminated structure uponoperation at a high temperature, fluctuating temperatures and/or under asubstantial temperature gradient. Another advantage of the combinedgelator systems used according to the present invention is that theirgelling kinetics can be modified, which opens for a delayedsignificantly slower gelling if so desired, this delay can in some casesexceed 24 h. This results in a decreased shrinkage than for a DC cablecomprising gelling composition according to the present invention. As aconsequence, the “post-filling” step is less critical.

According to one embodiment the dielectric gelling composition used asimpregnant in the DC-cable comprises a mineral oil and a combinedgelator system comprising a block copolymer and fine dielectricparticles. Particles with suitable electric and thermal properties havebeen found to be inorganic particles such as silica, alumina, zirconia,calcia and other oxides, silicon nitride, electrically insulating formsof carbon, zeolites, unexpanded and expanded mica, clays, talcs and thelike, coated particles comprising a coating of any of the materialsmentioned in the foregoing wherein the coating can be applied also onmetallic materials, e.g. fine particles of titanium coated with silicaand organic materials, such as cellulose based materials, e.g. cellulosepowder or micro-crystalline cellulose. The polymer can be polystyrene, adi- or tri block copolymer of styrene-butadiene-stryene orstyrene-ethylene/butylene-styrene. The cable can, when deemedappropriate, be complemented with reinforcing and a sealing compound ora water swelling powder for filling any interstices in and around theconductor, other metal/polymer interfaces may be sealed in order toprevent water from spreading along such interfaces.

A method for manufacture of an insulated electric device such as aDC-cable according to the present invention with an insulation systemimpregnated with a dielectric gelling composition comprising an oil anda gelator and exhibiting a thermo-reversible liquid-gel transition at atransition temperature, T_(t), wherein the gelling composition attemperatures below T_(t), is in a highly viscous elastic gelled statedand, at temperatures above T_(t), is in a liquid easy flowingessentially Newtonian state, comprises the steps of;

providing a conductor and a porous, fibrous and/or laminated structureof a solid electrically insulating material associated with each other;and

impregnating the porous, fibrous and/or laminated structure with adielectric fluid, and

gelling the dielectric gelling composition in the presence of a gelatorto impart the high viscosity and elasticity of a gel to the fluid at anyconditions for which the device is designed to operate under, wherein acombined gelator system comprising polymer molecules and fine dielectricparticles with a particle size in the nanometer range is prepared.Preferably the combined gelator system is added to the oil prior toimpregnation and the impregnation is carried out at a temperature abovethe transition temperature T_(t). According to one embodiment thepolymer molecules are grafted onto the fine dielectric particles.According to an alternative method the cable is, following impregnation,cooled to a temperature below T_(t), and following cooling a gellednetwork is formed in the gelling dielectric composition whereby theparticles are trapped in the gelled network. The particles shallpreferably be evenly distributed in the gelled network.

According to one embodiment the combined gelator system is added to theoil prior to impregnation and the impregnation is carried out at atemperature above the transition temperature T_(t), typically at atemperature of below 120° C., preferably at a temperature of from 50° C.to 120° C.

According to an alternative method the porous, fibrous and/or laminatedstructure is pretreated with the combined gelator system prior toimpregnation and the impregnation is carried out at a reducedtemperature, typically at a temperature of from 0° C. to 100° C.,preferably at a temperature of from 20° C. to 70° C. The woundinsulation can be soaked in or sprayed with a solution comprising agelator, dried and thereafter impregnated, but preferably it is woundfrom tapes that are already pretreated with gelling additives. The tapescan have been pretreated already in the line for tape production, butthe treatment can of course also have been done in a special treatmentoperation or in connection with the winding. This is the same for anytype of tape, such as an all paper tape, an all polymer tape or alaminated tape of paper and polymeric films or different polymeric filmsor meshes, webs or nets. Paper tapes can have been coated by spraying orimmersing or otherwise contacting the paper with a solution comprisingthe gelling additive. The gelling additive can have been added topolymeric films, tapes or the like by spraying or extruding the gellingadditive on to the polymer. A coating comprising the gelling additivecan also have been co-extruded with the polymeric tape or film. Thus,for a DC-cable comprising such a pretreated insulation, this embodimentwill ensure that the oil retains its easy flowing essentially Newtonianproperties during the essential period of filling phase of theimpregnation step and that the gelling additive thereafter, when broughtinto contact with the oil and at least in part dissolved by the oil,imparts the properties of a highly viscous, elastic gel to oil. Thetransformation of the easy flowing dielectric fluid to a highly viscousgel can dependent of the combination of gelling additive and dielectricfluid be instant, slow or even delayed. By instant transformation ismeant that the transformation is initiated directly as the gellingadditive is contacted and dissolved by the dielectric fluid and that thetransformation kinetics are such that the transformation is rapid. Theslow transformation is also typically initiated directly upon contactbetween fluid and gelling additive but the transformation is slowed downby the kinetics of the dissolution and/or transformation. A delayedtransformation for up to 24 hours can typically be accomplished by thegelling systems, gelator and matched oil, used in DC-cables according tothe present invention.

According to one further embodiment the impregnation is carried out inthe presence of a surfactant to further enhance the wetting duringimpregnation and thus provides opportunities for a shortenedimpregnation time and also for an improved oil penetration into smallvoids. The surfactant can either be added to the porous, fibrous and/orlaminated structure prior to impregnation by a pretreatment or it can bedissolved in the gelling composition prior to impregnation dependent ofwhich is deemed suitable from case to case.

According to one embodiment the different components of the combinedgelator system, i.e. the fine particles and the polymer compound areadded to different medium prior to impregnation. That is, the particlesare added to the solid part and the polymer to the oil or the particlesare added to the oil and the polymer to the solid part, whatever isfound suitable. Of course, the natural is way to add the combinedgelator system to either the solid part or the oil.

According to one further embodiment, the gelling additive is unevenlydistributed within the insulation such that it exhibits a concentrationgradient of the gelling additive that is increased inwards to theconductor. By distributing the gelling additive in this manner withinthe insulation several important aspects can be improved;

a more complete filling before start of gelling is ensured also for agelling system that gels almost instantly;

a self-healing capability is accomplished, i.e. a damaged part of theinsulation can be re-impregnated with fluid from other parts,

a gelled fluid that retains its highly viscous elastic gelled state alsowhen the temperature around the conductor is raised because of highloads used is obtained.

To ensure the long term stability-of the improved electrical andmechanical properties a gasabsorbing additive is included in theinsulating system. A suitable gasabsorbing additive is a low molecularpolyiosbutene with a molecular weight less than 1000 g/mole.

A DC-cable according to the present invention is ensured long termstable and consistent dielectric properties and a high and consistentelectric strength as good as, or better than for any conventionalDC-cable comprising such impregnated porous, fibrous and/or laminatedbody. This is especially important due to the long life suchinstallations typically are designed for, and the limited access formaintenance to such installations. The special selection and matching ofthe components in the combined gelator system, other additives, andoils, impregnants, ensure the long term stable properties of theinsulation system also when used at elevated temperatures, at excessivethermal fluctuations and/or under thermal gradients. This opens for acapability to allow an increase in the operation load both in regards ofincreased voltages and current densities. One further advantage of aDC-cable according to the present invention is that it, due to thesurfactant character of the gelators used in DC-cables according to thepresent invention, opens for a reduction in production time by enhancedwetting, which offers a possible shortened impregnation cycle. Also thetemperature sensitivity during production can be substantially reducedby a suitable selection and matching of oil and the components in thecombined gelator system which, opens for a delayed gelling, and therebyreduced sensitivity of the post-filling step.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention shall be described more in detail under referenceto the drawings and examples. FIG. 1 shows a cross-section of a typicalDC-cable for transmission of electric power comprising a wound andimpregnated insulation according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS, EXAMPLES

The DC-cable according to the embodiment of the present invention shownin FIG. 1 comprises from the center and outwards;

a stranded multi-wire conductor 10;

a first semi-conducting shield 11 disposed around and outside theconductor 10 and inside a conductor insulation 12;

a wound and impregnated conductor insulation 12 comprising a gellingadditive as described in the foregoing;

a second semi-conducting shield 13 disposed outside the conductorinsulation 12;

a metallic screen 14; and

a protective sheath 15 arranged outside the metallic screen 14. Thecable is further complemented with a reinforcement in form of metallic,preferably steel, wires outside the outer extruded shield 13, a sealingcompound or a water swelling powder is introduced in any interstices inand around the conductor 10.

The dielectric gelling composition of the present invention isapplicable for any arbitrary DC-cable with an insulation systemcomprising a solid porous or laminated part impregnated with adielectric fluid or mass. The application of the present invention isindependent of conductor configuration. It can also be used withDC-cables having an insulation system of this type comprising anyarbitrary functional layer(s) and irrespective of how these layers areconfigured. Its application to DC-cables of this type is alsoindependent of the configuration of the system for transmission ofelectric power in which the cable is included.

The DC-cable according to the present invention can be a singlemulti-wire conductor DC-cable as shown in FIG. 1, or a DC-cable with twoor more conductors. A DC-cable comprising two or more conductors can beof any known type with the conductors placed side-by-side in a flatcable arrangement, or in a two conductor arrangement with one firstcentral conductor surrounded by a concentrically arranged second outerconductor. The outer conductor is typically arranged in the form of anelectrically conductive sheath, screen or shield, typically a metallicscreen not restricting the flexibility of the cable.

A DC-cable according to the present invention is suitable for use inboth bipolar and monopolar DC-systems or installations for transmissionof electric power. A bipolar system typically comprises two or moreassociated single conductor cables or at least one multiconductor cable,while a monopolar installation has at least one cable and a suitablecurrent return path arrangement.

EXAMPLE 1

A gelling dielectric composition comprising a mineral oil and a combinedgelator system was prepared. The gelator system comprised polystyrenemolecules grafted or adsorbed onto silica particles with a particle sizein the nanometer range. The polystyrene molecules of the gelator systemwill thus interact with each other to develop a three dimensional,physically cross-linked network at temperatures below the transitiontemperature T_(t) 50-80° C. The bonds in this network are sufficientlystrong so that the composition at temperatures below T_(t) 50° C.behaves like an elastic or viscoelastic gel. A block of bundled porous,fibrous paper was impregnated with the gelling composition which, attemperatures up to 50° C., was fully retained in the porous, fibrousinsulation and between the paper layers.

EXAMPLE 2

The same gelling composition as prepared in example 1 was used toimpregnate a bundle of polypropen films, where the films were of thesolid type. The gelling composition was fully retained between the filmlayers in the laminated insulation.

EXAMPLE 3

The same gelling composition as prepared in example 1 was used toimpregnate a bundle of laminated polypropen-paper sheets, where eachsheet comprises a polypropen film of the solid type laminated with apaper film. The gelling composition was fully retained in the paper partof the insulation and between the laminated layers.

EXAMPLE 4

A gelling dielectric composition comprising a mineral oil and a combinedgelator system was prepared. The gelator system comprisedstyrene-butadiene-styrene di block copolymer molecules grafted oradsorbed onto silica particles with a particle size in the nanometerrange. The polystyrene molecules of the gelator system will thusinteract with each other to develop a three dimensional, physicallycross-linked network at temperatures below the transition temperatureT_(t) 50° C. The bonds in this network are sufficiently strong so thatthe composition at temperatures below T_(t) 50° C. behaves like anelastic or viscoelastic gel. A block of bundled porous, fibrous paperwas impregnated with the gelling composition which, at temperatures upto 50° C., was fully retained in the porous, fibrous insulation andbetween the paper layers.

EXAMPLE 5

The same gelling composition as prepared in example 4 was used toimpregnate a bundle of polypropen films, where the films were of thesolid type. The gelling composition was fully retained between the filmlayers in the laminated insulation.

EXAMPLE 6

The same gelling composition as prepared in example 4 was used toimpregnate a bundle of laminated polypropen-paper sheets, where eachsheet comprises a polypropen film of the solid type laminated with apaper film. The gelling composition was fully retained in the paper partof the insulation and between the laminated layers.

EXAMPLE 7

A gelling dielectric composition comprising a mineral oil and a combinedgelator system was prepared. The gelator system comprisedstyrene-ethylene/butylene -styrene tri block copolymer molecules graftedor adsorbed onto silica coated titanium particles with a particle sizein the nanometer range. The polystyrene molecules of the gelator systemwill thus interact with each other to develop a three dimensional,physically cross-linked network at temperatures below the transitiontemperature T_(t) 50-80° C. The bonds in this network are sufficientlystrong so that the composition at temperatures below T_(t) 50° C.behaves like an elastic or viscoelastic gel. A block of bundled porous,fibrous paper was impregnated with the gelling composition which, attemperatures up to 50° C., was fully retained in the porous, fibrousinsulation and between the paper layers.

EXAMPLE 8

The same gelling composition as prepared in example 7 was used toimpregnate a bundle of polypropen films, where the films were of thesolid type. The gelling composition was fully retained between the filmlayers in the laminated insulation.

EXAMPLE 9

The same gelling composition as prepared in example 7 was used toimpregnate a bundle of laminated polypropen-paper sheets, where eachsheet comprises a polypropen film of the solid type laminated with apaper film. The gelling composition was fully retained in the paper partof the insulation and between the laminated layers.

EXAMPLE 10

Examples 1 to 9 were repeated, except for using zeolite particles inplace of the silica particles and silica coated titanium particles, withsimilar good results. The transition temperature was in the range 50-80°C.

These blends of the examples referred to exhibit a development of astable network and a high temperature liquid-gel transition. The resultsof these examples have shown it probable that with these gelators addedto an oil used for impregnation of a conductor insulation in a DC-cableaccording to the present invention, faster impregnation rates and lowerimpregnation temperatures can be employed compared to conventionallyused gelling impregnants. Further, the retention test described in theexamples shows that the gelling compositions at temperatures below T_(t)behave like elastic bodies and that the oil is at these temperaturesfully retained in the porous, fibrous insulation and between thelaminated layers. Repeating this last test for oil retention for aconventionally used insulating oil show a slows flow of oil, leaking outfrom the bundled block. Thus, the risk for voids appearing duringoperation is drastically reduced and the electrical properties of theconductor insulation in a device according to the invention areimproved. The improvements related to in the foregoing are likely toresult in a cable comprising a wound paper-insulation impregnated withthe dielectric system described in the foregoing where essentially allvoids in the insulation are filled by the dielectric impregnant, i.e.the insulation is essentially fully impregnated. Such a cable is alsolikely to, after use at elevated temperatures and high electrical,essentially static fields, exhibit a low number of unfilled voids andthus to be less sensitive to dielectric breakdown.

What is claimed is:
 1. A high voltage electric cable for transmission ordistribution of electric power having at least one conductor and animpregnated insulation system comprising a solid electrically insulatingdielectric part with a porous, fibrous and/or laminated structureimpregnated with a dielectric gelling composition comprising an oil anda gelator and having a thermo-reversible liquid-gel transition at atransition temperature, T_(t), wherein the gelling composition attemperatures below T_(t) has a first viscosity and, at temperaturesabove T_(t), has a second viscosity which is less than the firstviscosity, the gelator comprises a combined gelator system havingmolecules of a polymer compound, said compound comprising a polarsegment capable of forming hydrogen bonds, together with fine dielectricparticles having a particle size of less than 1000 nm.
 2. A high voltageelectric cable according to claim 1, wherein the fine dielectricparticles have a particle size in the range of from 1 to 1000 nm.
 3. Ahigh voltage electric cable according to claim 2, wherein the finedielectric particles have a particle size in the range of from 10 to 100nm.
 4. A high voltage electric cable according to claim 1, wherein thepolymer compound and the oil interact to develop a three dimensional,physically cross-linked gelled network at temperatures below thetransition temperature T_(t).
 5. A high voltage electric cable accordingto claim 4, wherein the fine dielectric particles are trapped within agelled network of polymer.
 6. A high voltage electric cable according toclaim 5, wherein the fine dielectric particles are physically bonded tothe gelled network of polymer.
 7. A high voltage electric cableaccording to claim 1, wherein the polymer molecules are grafted onto thefine particles.
 8. A high voltage electric cable according to claim 1,wherein the fine dielectric particles are evenly distributed within agelled network of polymer.
 9. A high voltage electric cable according toclaim 1, wherein the transition temperature T_(t), is a narrow range oftemperatures above 30° C.
 10. A high voltage electric cable according toclaim 9, wherein the transition temperature ranges from 50° C. to 120°C.
 11. A high voltage electric cable according to claim 1, wherein thefine dielectric particles comprise cellulose based particles.
 12. A highvoltage electric cable according to claim 11, wherein the finedielectric particles comprise micro crystalline cellulose.
 13. A highvoltage electric cable according to claim 1, wherein the fine dielectricparticles comprise electrically insulating inorganic particles.
 14. Ahigh voltage electric cable according to claim 13, wherein the finedielectric particles comprise a metal oxide.
 15. A high voltage electriccable according to claim 14, wherein the fine dielectric particlescomprise silica.
 16. A high voltage electric cable according to claim 1,wherein the fine dielectric particles comprise a zeolite.
 17. A highvoltage electric cable according to claim 1, wherein the fine dielectricparticles comprise a clay.
 18. A high voltage electric cable accordingto claim 1, wherein the fine polymer compound comprises polar segmentsand linear non-polar hydrocarbon chains soluble in the dielectricgelling composition.
 19. A high voltage electric cable according toclaim 1, wherein the polymer compound comprises a sugar based compound.20. A high voltage electric cable according to claim 1, wherein thepolymer compound comprises urea or di-urea.
 21. A high voltage electriccable according to claim 1, wherein the polymer compound comprises ablock copolymer.
 22. A high voltage electric cable according to claim 1,wherein the polymer compound comprises a polyalkylsiloxane.
 23. A highvoltage electric cable according to claim 1, wherein the polymercompound comprises a cellulose based compound.
 24. A high voltageelectric cable according to claim 1, including a surfactant.
 25. Aninsulated electric device according to claim 1, wherein the dielectricparticles at temperatures below T_(t) are trapped within a gellednetwork.
 26. A high voltage electric cable according to claim 1, whereinthe dielectric gelling composition interacts with the surface of theporous, fibrous and/or laminated structure.
 27. A high voltage electriccable according to claim 1, wherein the dielectric gelling compositioncomprises a mineral oil and a combined gelator system comprising a blockcopolymer and fine dielectric particles.
 28. A high voltage electriccable according to claim 1, wherein the dielectric gelling compositioncomprises a mineral oil and a gelator system comprising a blockcopolymer that comprises an olefin based block and one block witharomatic rings in its backbone structure.
 29. A high voltage electriccable according to claim 1, wherein the dielectric gelling compositioncomprises a polystyrene.
 30. A high voltage electric cable according toclaim 1, wherein the dielectric gelling composition comprises astyrene-ethylene/butylene-styrene triblock copolymer.
 31. A high voltageelectric cable according to claim 1, wherein the dielectric gellingcomposition comprises a styrene-butadiene-styrene triblock polymer. 32.A method of manufacturing a high voltage electric cable according toclaim 1 comprising: providing a conductor and a porous, fibrous and/orlaminated structure of a solid electrically insulating materialassociated with each other; and impregnating the porous, fibrous and/orlaminated structure with a dielectric fluid, and gelling the dielectricgelling composition in the presence of a gelator to impart a viscosityof a gel to fluid at any condition for which the high voltage electriccable is designed to operate under, wherein a combined gelator systemcomprising polymer molecules of a polymer compound, said compound beingselected from polymer compounds comprising a polar segment capable offorming hydrogen bonds, a sugar based compound, urea or di-urea, a blockcopolymer, a polyalkylsiloxane, a cellulose based compound, togetherwith fine dielectric particles based on dielectric organic or inorganicmaterials, or any particles coated with such material, said particles aparticle size of less than 1000 nm, is prepared.
 33. A method accordingto claim 32, wherein the combined gelator system is added to the oilprior to impregnation and that the impregnation is carried out at atemperature above the transition temperature T_(t).
 34. A methodaccording to claim 32, wherein the polymer molecules are grafted ontothe fine dielectric particles.
 35. A method according to claim 32,wherein following impregnation the cable is cooled to a temperaturebelow T_(t), and that following cooling a gelled network is formed inthe gelling dielectric composition whereby the fine dielectric particlesare trapped in the gelled network.
 36. A method according to claim 35,wherein the fine dielectric particles are evenly distributed in thegelled network.
 37. A method according to claim 32, wherein theimpregnation is carried out at a temperature below 120° C.
 38. A methodaccording to claim 37, wherein the temperature ranges from 50° C. to120° C.
 39. A method according to claim 32, wherein the porous, fibrousand/or laminated structure is pretreated with the combined gelatorsystems prior to impregnation and that the impregnation is carried outat a reduced temperature.
 40. A method according to claim 39, whereinthe impregnation of the pretreated structure is carried out at atemperature of from 0° C. to 100° C.
 41. A method according to claim 40,wherein the temperature ranges from 20° C. to 70° C.
 42. A methodaccording to claim 32, where the impregnation is carried out in thepresence of a surfactant.
 43. A method according to claim 42, whereinthat the porous, fibrous and/or laminated structure is pretreated withthe surfactant prior to impregnation.
 44. A method according to claim42, wherein that the surfactant is dissolved in the gelling compositionprior to impregnation.
 45. A method of manufacturing a high voltageelectric cable for transmission or distribution of electric powercomprising a dielectric gelling composition comprising an oil and agelator and having a thermo-reversible liquid-gel transition at atransition temperature, T_(t), wherein the gelling composition attemperatures below T_(t) has a first viscosity and, at temperaturesabove T_(t), has a second viscosity which is less than the firstviscosity, wherein the method comprises: providing a conductor and aporous, fibrous and/or laminated structure of a solid electricallyinsulating material associated with each other; impregnating the porous,fibrous and/or laminated structure with a dielectric fluid; and gellingthe dielectric gelling composition in the presence of a gelator toimpart a viscosity of a gel to the fluid at any conditions for which thedevice is designed to operate under, wherein a combined gelator systemof polymer molecules exhibiting a polar segment capable of forminghydrogen bonds molecules and fine dielectric particles with a particlesize of less than 1000 nm is prepared.